Communication systems evolve more and more towards an Internet Protocol IP-based network. They consist of many interconnected networks, in which speech and data are transmitted from one terminal to another terminal in pieces, so-called packets. Those packets are routed to the destination by routers in a connection-less manner. Therefore, packets consist of IP header and payload information and the header comprises among other things a source and destination IP address. For scalability reasons an IP network uses a hierarchical addressing scheme. Hence, an IP address does not only identify the corresponding terminal, but additionally contains information about the topological location of the terminal. With additional information provided by routing protocols, routers in the network are able to identify the next router towards a specific destination.
If a mobile terminal, which will be defined by the term Mobile Node (MN) in the present application, moves between subnets, it must change its IP address to a topological correct one because of the hierarchical addressing scheme. However, since connections on higher-layers such as TCP connections are defined with the IP addresses (and ports) of the communicating nodes, the connection breaks if one of the nodes changes its IP address, e.g. due to movement.
An adapted IP protocol has been proposed, referred to by the term Mobile IPv6, that describes an IP-based mobility protocol enabling Mobile Nodes to move between subnets in a manner transparent for higher layers and applications, i.e. without breaking higher-layer connections. Therefore, a Mobile Node has two IP addresses configured: a Care-of-Address (CoA) and a Home Address (HoA). The Mobile Node's higher layers use the Home Address for communication with the communication partner (destination terminal), which will be defined by the term Corresponding Node (CN) in the present application. The Home Address does not change and serves the purpose of identification of the Mobile Node. Topologically, it belongs to the Home Network (HN) of the Mobile Node. In contrast, the Care-of-Address changes on every movement resulting in a subnet change and is used as the locator for the routing infrastructure. Topologically, it belongs to the network the Mobile Node is currently visiting. An Home Agent (HA) located on the home link maintains a mapping of the Mobile Node's Care-of-Address to the Mobile Node's Home Address and redirects incoming traffic for the Mobile Node to its current location.
Mobile IPv6 currently defines two modes of operation: bi-directional tunneling and route optimization. If bi-directional tunneling is used, data packets sent by the Corresponding Node and addressed to the Home Address of the Mobile Node are intercepted by the Home Agent in the Home Network and tunneled to the Care-of-Address of the Mobile Node. Data packets sent by the Mobile Node are reverse tunneled to the Home Agent, which decapsulates the packets and sends them to the Corresponding Node. For this operation, only the Home Agent must be informed about the Care-of-Address of the Mobile Node. Therefore, the Mobile Node sends Binding Update (BU) messages to the Home Agent. These messages are sent over an IPsec security association and thus are authenticated and integrity protected. One drawback of this mode is that if the Mobile Node is far away from the Home Network and the Corresponding Node is close to the Mobile Node, the communication path is unnecessarily long, resulting in inefficient routing and high packet delays.
The route optimization mode can prevent this inefficiency by using the direct path between Corresponding Node and Mobile Node. Therefore, the Mobile Node sends Binding Update messages to the Corresponding Node, which then is able to directly send data packets to the Mobile Node (a type 2 routing header is used to send the packets on the direct path). Of course, the Corresponding Node has to implement Mobile IPv6 route optimization support. To authenticate the BU message, the Mobile Node and the Corresponding Node perform a so-called return routability procedure, which tests the reachability of the Mobile Node at the Home Address and Care-of-Address and generates a shared session key.
A standard Mobile IP handover procedure is illustrated in FIG. 1 and will now be described. A standard Mobile IP handover procedure comprises many steps. First, the Mobile Node needs to detect a new layer 2 link. Then, it can execute the layer 2 handover. After that, it needs to detect the layer 3 link change. In the next step, it has to discover the new layer 3 prefix and the new Access Router (AR) address. Subsequently, it must obtain a new Care-of-Address, possibly using stateless auto-configuration, which also includes a Duplicate Address Detection (DAD) procedure. Then it must send a BU message to the Home Agent and the Corresponding Node before IP packets can finally be delivered to the Mobile Node at the new location.
In standard Mobile IPv6, a layer 3 link change is detected, e.g. when a certain number of Router Advertisement (RA) messages are not received, which are usually sent periodically. A timer can be used to check this condition. Even with the lowest allowed interval of sending RA messages (30 ms) and with a threshold of three missed advertisements, this step takes at least 90 milliseconds. The Duplicate Address Detection procedure is also based on a timer and requires the node to send a Neighbour Solicitation (NS) message containing the tentative address and waiting for a response from a node with the same address. If no response is received after three trials, the address is considered unique. A Mobile IP handover procedure may thus take a long time, which can render the use of delay-sensitive communication applications impossible.
Many optimizations have been proposed to improve the Mobile IP handover delay. For instance, the use of multiple interfaces can significantly reduce the handover delay, since data can be sent and received over the first interface while the handover procedure on the second interface is in progress. This requires multiple interfaces with the same or similar characteristics, such as delay, Quality-of-Service support, etc. However, some terminals are not equipped with multiple such interfaces and the integration of a second interface only for enabling fast handovers may be too expensive.
Another way to reduce handover delay is to increase the frequency with which routers send Router Advertisement messages. This way, the Mobile Node can detect a layer 3 link change more quickly. However, the amount of signalling over the air would increase significantly and the remaining handover delay would still be high.
Another possible way is to multicast data to the current and neighbouring Access Router. However, this only reduces the delay during BU latency and would result in many unnecessary data packets going over the network.
The IETF Detecting Network Attachment Working Group has been working on solutions to quickly detect a layer 3 link change. Solutions are proposed for modified and unmodified routers. The former allows faster handovers than the latter and consists of two parts. First, a Mobile Node sends a Router Solicitation (RS) message immediately after it has completed its layer 2 handover. This requires hints from layer 2. The second part enables the detection of a layer 3 link change based on a single received Router Advertisement. Therefore, this Router Advertisement may contain a complete prefix list, which can be compared with the prefix of a previously received Router Advertisement to decide whether the Mobile Node has changed its layer 3 link. To be able to construct the complete prefix list, every router learns the prefixes from other routers on the link from received Router Advertisement messages. However, a major drawback is that the Router Advertisement messages can get big and signalling overhead over the air can get high, if many different routers and prefixes exist on a link.
To mitigate this problem, two optional mechanisms have been introduced. First, the “landmark” option allows the Mobile Node to ask a router if a certain prefix exists on the link. This is done with the Router Solicitation message. A router then replies with yes or no in a Router Advertisement message. The “single prefix” option prescribes that routers on the same link agree on a certain prefix that is always contained in a Router Advertisement. Hence, at least this prefix should be different from a Router Advertisement sent by a router on a different link. The time for layer 3 link change detection, new Access Router discovery, layer 3 prefix discovery, and Duplicate Address Detection can be significantly reduced. However, the time for layer 2 handover, Care-of-Address construction and Binding Update latency remains and packet loss may occur during this time. Also, the layer 3 link detection is not zero, since the Mobile Node needs to send a Router Solicitation before receiving a Router Advertisement.
Another approach is followed by the IETF Network-based Localized Mobility Management (NETLMM) Working Group. The idea is to let the network, i.e. the Access Routers, handle the mobility transparently for the Mobile Node. Hence, the Mobile Node does not detect layer 3 link changes and does not need to change its IP address. This saves the delay of layer 3 link change detection (assuming the existence of layer 2 triggers), new Access Router discovery, new Care-of-Address construction, layer 3 prefix discovery, and Duplicate Address Detection. Binding Update latency is reduced by introducing a local mobility agent, the so-called Mobility Anchor Point (MAP). However, those benefits are only obtained for handovers within a limited area, referred to as the NETLMM domain, which might be a part of an operator's network domain. Packet loss may still occur during layer 2 handover and all the benefits are only obtained as long as the Mobile Node does not leave the NETLMM domain.
In [R. Koodli, “Fast Handovers for Mobile IPv6”, IETF RFC 4068, July 2005], multiple methods for fast handovers are specified. First, a Mobile Node can obtain a prefix and IP address of the new Access Router before starting the layer 2 handover. This anticipation is achieved by allowing the Mobile Node to send a Proxy Router Solicitation message to the old Access Router, which contains the layer 2 address of the new Access Router, i.e., handover prediction is required. The network can then determine the new Access Router address and prefix and inform the Mobile Node about it. Hence, the Mobile Node can construct the new Care-of-Address while still being at the old link.
In the so-called predictive mode, the Mobile Node sends a Fast Binding Update to the Access Router on the old link. This Access Router then communicates with the new Access Router to check uniqueness of the new Care-of-Address and to setup a tunnel for forwarding the Mobile Node's traffic. While the Mobile Node is in the process of layer 2 handover, the new Access Router can buffer incoming packets and when the Mobile Node has completed the layer 2 handover, the new Access Router can immediately deliver packets to the Mobile Node. In the so-called reactive mode, the Fast Binding Update is sent after the layer 2 handover on the new link. Hence, Duplicate Address Detection and tunnel establishment is happening after the Mobile Node has completed the layer 2 handover.
In the predictive mode, packet loss can be prevented and packets can be delivered to the Mobile Node immediately after completing the layer 2 handover. However, a problem exists if there is not enough time before the layer 2 handover to send the Fast Binding Update message. In this case, only the reactive mode can be used. However, the reactive mode can result in significant handover latency, since Duplicate Address Detection and tunnel establishment happen in the critical phase after the layer 2 handover. Moreover, if there is not enough time to send Proxy Router Solicitation messages before the layer 2 handover or if the prediction was wrong, i.e. the Mobile Node hands over to another Access Router, none of these methods can be used. Another drawback is the high amount of signaling over the air involved.
U.S. Pat. No. 6,978,137 proposes a method for handover prediction, in which both Mobile Node and network send the output of their handoff prediction to a central server, which triggers handoff if both predictions match. This method can be used in anticipated handover methods, but cannot reduce handover latency by itself.
WO2005053187 describes a method for fast handovers that shifts the layer 3 link change, Duplicate Address Detection and Care-of-Address generation functionality from the Mobile Node to the Access Router. After a link layer handover, a Mobile Node sends a modified Router Solicitation message to the Access Router, which can then detect a layer 3 handover, generate a Care-of-Address, perform Duplicate Address Detection and transmit this Care-of-Address to the Mobile Node in a modified Router Advertisement message. However, this method requires modifications to the Mobile Node and Access Router implementations. Furthermore, it does not eliminate handover latency, since a round trip time to the Access Router is required for the Router Solicitation/Router Advertisement exchange and the Binding Update transmission latency remains.